One of the primary problems limiting the performance of diode-pumped solid state lasers is the ability to remove heat generated within the lasing medium from the pump energy absorbed. Since only a small part of the pump energy is converted into laser energy, usually significantly less than 20%, the majority is absorbed and converted into heat, and has to be removed from the lasing medium. If the heat is not removed efficiently, the temperature of the lasing medium rises, thereby degrading the efficiency of the lasing action. Furthermore, if the temperature is not uniformly distributed within the medium, which can result, inter alia, from poor cooling design, aberrational thermal lensing can result, causing degradation and undetermined change of the beam mode. In addition, the lasing medium can undergo severe mechanical stress, causing distortion of the laser beam and even a danger of rod fracture. Efficient cooling of the lasing medium, on the other hand, enables higher lasing power to be extracted, and with a higher quality beam.
For these reasons, a great deal of effort has been expended on the development of efficient methods of cooling the lasing medium in solid state lasers, and especially, because of their more concentrated pump input levels, in diode pumped solid state lasers. Commonly used prior art cooling methods for end-pumped lasers include:    (a) The use of flowing water in direct contact with the outer envelope of the laser rod. Throughout this application, and as claimed, the term “rod” in connection with the lasing medium is understood to include “slab”, or any other suitable lasing medium geometry.    (b) The application of water-cooled blocks of a high conductivity metal, such as copper, in intimate thermal contact with the lasing rod or slab, usually with a very thin intermediate layer of indium, typically 100 μm thick, to improve thermal contact.    (c) The application to the pumped face of a thin sapphire plate, whose periphery is cooled, to conduct the heat away from the face.
It is well known that cooling of a lasing slab in the same direction as the direction of pumping is generally desirable, since the temperature gradient generated by the heat dissipation is then in the same direction as the axis of cooling. As a result, for a side-pumped slab, geometrical symmetry of the temperature equipotentials is maintained with respect to the cooling/pumping direction. The lasing axis is perpendicular to this direction, along the length of the slab, and the repeated traverses of the intra-cavity beam following a zigzag geometrical path, ensure that the beam wavefront passes equally through all parts of the slab, such that on average, the beam undergoes no lensing in the zigzag direction. In the direction of the slab cross-section perpendicular to the zigzag direction, there is no such averaging, but since the temperature equipotentials are constant in this direction, there is no directional effect on the beam as it passes through each “temperature layer”, and thermal lensing is thus minimized.
There are disadvantages to each of the above methods, as follows:    (a) The use of direct water-cooling of the laser rod, though very efficient, significantly increases the complexity of the head design. Firstly, there is a need to provide water seals at the relevant locations, with all the concomitant mechanical complexity. Secondly, there is need to prevent corrosion because of the water. The head design complexity is further compounded when the head is such that it is pumped through the face to be cooled, which, as explained above, is the preferred configuration. Finally, if the water flow is in direct contact with the laser rod, turbulence created by the water flow can cause the rod to vibrate, thereby causing sympathetic beam vibration or wander.    (b) The use of a water-cooled copper heat sink (or any other conductive metal) is thermally efficient, because of the high conductivity of the copper. The major disadvantage of this method, though, is that the opacity of the copper does not enable pumping of the laser in the same direction as the cooling direction, as preferred. Another disadvantage is the need to coat the copper block with an inert material such as gold, to avoid contamination or corrosion resulting from the comparatively “dirty” chemical nature of the copper.    (c) There is a disadvantage in the use of a peripherally-cooled sapphire cooling plate in that sapphire has a limited thermal conductivity, and this limits the pump load that can be born by the laser rod. For this reason, a peripherally-cooled sapphire cooling plate needs to be cooled as close as possible to the point of contact with the lasing medium. For example, in the above-mentioned Weber article, the sapphire plate is used to cool only the end face of the laser rod, the end face being of small dimensions, such that the heat flow path through the sapphire plate is of minimal length. The length of the rod then also needs its own water cooling arrangement.
Sapphire plates have also been described in the prior art in use in cooling side pumped lasers. In U.S. Pat. No. 5,974,061, to R. W. Byren et al., an edge pumped laser is described, in which sapphire plates are used as cladding to the wide side of the laser slab, while the slab is side pumped through its narrow edge. The sapphire cladding area is cooled by direct contact with water cooled copper or aluminum blocks, such that the heat flow is across the thickness of the sapphire regions, and similar to the disadvantages mentioned in paragraph (b) above, the pumping cannot be performed through the sapphire-cooled regions.
In U.S. Pat. No. 5,790,575 to J. M. Zamel et al., another side-pumped laser using sapphire plates is described, with pumping directed through the sapphire plates. However, in this laser, the sapphire plates act as transparent side walls for a water channel disposed between the lasing slab and the sapphire plates, and the lasing slab is effectively cooled by direct contact with flowing water. This design therefore possesses all the disadvantages mentioned in paragraph (a) above, of mechanical complexity and of the effects of water turbulence.
In U.S. Pat. No. 5,317,585, to E. Gregor, there is described a side pumped solid state laser, incorporating a thin sapphire heat conducting plate, which conducts heat away from the region of the lasing slab to heat sinks located at the peripheries of the plates. The laser is pumped along the same axis as that of the heat transfer from the slab. The heat conduction path in the sapphire plate is short, but because of the limited thermal conductivity of the sapphire, it would appear unlikely that the laser can be used at the maximum powers which its lasing slab would be capable of achieving if it were cooled more efficiently. This assumption is supported by some other described features of the laser, from which it is apparent that optimum heat conduction was not used. Thus for instance, the thermal contact of the sapphire plate(s) with the lasing slab is not optimized, being executed through “layers of transparent elastic material”, which inevitably have a compromised value of thermal conductivity in comparison with solid, heat conductive, materials. In another embodiment, with a cylindrical rod geometry, thermal contact between the lasing rod and the sapphire is achieved by means of a cooling channel which contains slow flowing or even static water.
It is therefore expected that the use of sapphire heat conduction plates will result in significant lateral temperature gradients along the length of the sapphire plates, and hence, poor overall cooling, and poor uniformity of cooling of the lasing slab.
There therefore exists a serious need for a method of cooling side-pumped and end-pumped diode-pumped solid state lasers, which overcomes the various disadvantages and drawbacks of the prior art cooling methods.